Typical internal combustion engines have many reciprocating parts for each combustion chamber, such as pistons, piston rods, and valve assemblies. Because these reciprocating parts are constantly accelerated and decelerated while the engine is at work, they rob output power from the engine and cause vibrations which can damage the engine or create a nuisance.
Prior engines also have many parts which rotate while the engine is in motion. Such parts include a crankshaft and flywheel, fan, camshaft, and other related parts of the engine. These engine parts have substantial spin angular momenta and thus are like gyroscopes, preventing the engine mounting from being moved freely. If the engine is mounted to a vehicle, this gyroscopic effect can inhibit steering.